A heat exchanger using an aluminum product is widely used as a heat exchanger for a radiator, a car heater, an oil cooler, and an intercooler of a motor vehicle, an evaporator and a condenser of an automotive air conditioner, or a heat exchanger for hydraulic equipment and an industrial machine.
In recent years, since a heat exchanger tends to be reduced in weight from the view point of improving fuel consumption and space saving, the members used therein are required to have thin-wall high strength and high corrosion resistance. Particularly, since a tube material is used in a large amount, these requirements are high. Further, recently, uneven processing is sometimes applied to a formed tube for improving heat exchanging performance, and the processed portion may undergo large deformation accompanied by reduction in thickness. In this case, if the elongation of material is low, there is a problem that the material breaks during processing, or it is difficult to obtain a desired shape. To cope with such a problem, Patent Literature 1, for example, proposes an improvement in formability by transforming a sacrificial material and a core material into fibrous structure to equalize the deformability of the sacrificial material and the core material for the purpose of improving formability.